CH 4 Lecture Slides: Newton's second law of motion

force causes

acceleration

acceleration is

directly proportional to net force

acceleration equation

→a = →F net/m

force types

• push and pull

• gravity (downward)

• tension from ropes

• supporting force (normal force)

• frictional forces

friction between solid surfaces

is due to tiny surface bumps and to “stickiness” of the atoms on a material’s surface

frictional force equation

(coefficient of frictional force) (normal force)

drag force

depends on front surface area and how fact the object moves

mass

• quantity of matter in an object

• the measure of inertia that an object exhibits in response to any effort made to start it, stop it, or change its motion

• measured in kilogram (Kg)

weight

• the force upon an object due to gravity

• the scientific unit of force is the Newton (N)

• unit is also the pound (lb)

mass

resists acceleration

the same force applied to twice the mass produces

half the acceleration

the same force applied to three times the mass produces

1/3 the acceleration

the greater the mass of the object the

• greater its force of attraction toward the Earth, the smaller its tendency to move, the greater its inertia

• acceleration is the same

newton’s second law provides an explanation for the

equal accelerations of freely falling objects of various masses

acceleration is equal when

air resistance is negligible

acceleration depends on

force (weight) and inertia

when an object falls downward through the air it experiences

• force of gravity pulling it downward

• air drag force, R, acting upward

air drag force

• R

• depends on the shape and falling speed of the object

when air resistance is non-negligible the

acceleration of fall is less than g (a<g, non free fall)

air resistance depends on

speed and surface area

terminal velocity

when the object is moving fast enough that force of gravity equals its air resistance

newton’s second law

internal forces cannot make the object to move

newton’s second law equation

F net = ma

F net is the

sum of the external forces

A 5kg iron ball and a 10kg iron ball are dropped from rest at the same location. For negligible air resistance, the acceleration of the heavier ball will be

the same

When a 20-N falling object encounters 5N of air resistance, it acceleration of fall is

less than g

If a 50 N person is to fall at terminal speed, the air resistance needed is

50 N

As the skydiver falls faster and faster throughout the air, air resistance

increases

As the skydiver falls faster and faster throughout the air, net force

decreases

As the skydiver falls faster and faster throughout the air, her acceleration

decreases

Consider a heavy and light person jumping together with the same-size parachutes from the same altitude. Who will reach the ground first? 

the heavy person

A coin and a feather are dropped at the same time inside a tube filled with air, which one reaches the bottom of the tube first?

the coin

A coin and feather are dropped at the same time inside a tube under a vacuum, which one reaches the bottom of the tube first?

They will reach the bottom at the same time

When Sanjay pushes a refrigerator across a kitchen floor at a constant speed, the force of friction between the refrigerator and the floor is

equal and opposite to Sanjay’s push

When Sanjay pushes a refrigerator across a kitchen floor at an increasing speed, the force of friction between the refrigerator and the floor is

less than Sanjay’s push